Removal and separation of metallic oxide scale

ABSTRACT

A method of removing metallic oxide scale from the cooling system of a nuclear reactor is disclosed. By forming complexes of the metals using ammoniated citric acid, the scale can be removed. By selectively adsorbing these complexes on ion exchange resins the scale can be separated.

Elnited States Patent Oberhofer et al.

[451 May 23, 1972 154] REMOVAL AND SEPARATION OF METALLIC OXIDE SCALE[72] Inventors: Allred W. Oberhofer, Alsip; Donald R. Anderson, Oswego;Charles C. Payne, Chicago, all of Ill.

[73] Assignee: Nalco Chemical Company, Chicago, 111.

[22] Filed: Oct. 29, 1969 [21] Appl. No.: 872,411

52 use ..134/3,134/10,134/22, 210/38 51 lm.Cl. ..-.....C23gl/02,C23gl/365s FieldofSearch ..134 3,22,|0,13,27,2s, l34/29,41;2l0/38 [56]References Cited UNITED STATES PATENTS 3,013,909 12/1961 Pancer et a1. 134/3 Primary ExaminerMorris O. Wolk Assistant Examiner-Joseph T. ZatargaAtt0rneyJohn G. Premo, Charles W. Connors and Edward A. Ptacek 57ABSTRACT A method of removing metallic oxide scale from the coolingsystem of a nuclear reactor is disclosed. By forming complexes of themetals using ammoniated citric acid, the scale can be removed. Byselectively adsorbing these complexes on ion exchange resins the scalecan be separated.

5 Claims, No Drawings REMOVAL AND SEPARATION OF METALLIC OXIDE SCALEINTRODUCTION Usually, boiler scale consists of metal carbonates,sulfates, and similar well-known salts. The scale in the cooling systemof a nuclear reactor consists essentially of metallic oxides, althoughvery minor amounts of other compounds could be present. These metallicoxides consist of copper, cobalt, and iron oxides. Minor substituentscould be manganese, chromium, nickel, vanadium and titanium. The waterused in the cooling system of a nuclear reactor is very pure. Thisaccounts for the general absence of sulfates and carbonates.

The metallic oxide scale buildup in the cooling system of a nuclearreactor is not only significant but also difficult to cope with due tothe radioactivity. If a process is devised for dissolving the crud orscale buildup inside the system, there is still the problem what to dowith the radioactive crud. It cannot safely be disposed ofin our wastestreams.

A need exists for an improved method of removing scale and foreignmaterial from the interior surfaces of such a system and concentratingthe radioactive contaminants. It would be a benefit to the art if thelong-life radioactive contaminants could be separated from theshort-life radioactive contaminants. The necessary precautions could bepracticed with the long-life radioactive contaminants, while theshortlife contaminants would present less of a problem.

OBJECTS It is an object of this invention to provide a method for easilydissolving, removing, and separating the metallic oxide scale or crud ofthe cooling system ofa nuclear reactor.

It is a further object of this invention to provide a method forseparating the longand short-life radioactive substances of the crudfrom the nuclear reactor cooling system.

Other objects will be apparent to those skilled in the art.

THE INVENTION The invention comprises chelating the metal compoundswhich make up the metallic oxide scale to form soluble complexes andremoving these complexes from solution using ion exchange resins andappropriate pH adjustments.

Generally this invention involves a number of steps. First, a solutionof chelating agent is added to the nuclear reactor cooling system. Thesolution of the chelating agent can range from 0.5-6 percent. The pHofthe solution can range from 1.9 to 3.5 and preferably is about 3.Iron, cobalt, and some copper are then dissolved from the interiorsurfaces of the system containing the scale using the chelatingsolution. The temperature can range from 70 to 225 F., but preferably ismaintained at 200 F.

The second step involves passing the solution at a temperature of 175 F.through a cation exchange resin. The cobalt complex is removed fromsolution in this step. Preferably the solution is then passed throughanother cation exchange resin to remove any traces of cobalt that remainin the solution. This is a polishing column. I

Thirdly, the solution is then adjusted by the addition of chloride-freeammonia to a pH of from -7 and preferably a pH offrom 5.5-6.5.

Fourthly, the solution is passed through a column containing an anionexchange resin. In this step most of the iron is removed and some of thecopper. Preferably this step is followed by passing the solution throughanother anion exchange resin which removes any traces of iron stillpresent in the solution.

Since the iron and the cobalt have been removed from the scale of thesystem, the fifth step is to remove the copper. The temperature of thesolution is then about 150 F. The pH is adjusted using chloride-freeammonia to from 9-11. The solution is returned to the system to dissolveany remaining scale. Some of the citric acid is removed with the iron.Therefore, the solution now contains less citrate and more ammonia thanpreviously. Thus, the solution is now more condusive for the dissolutionof the copper. Now that the solution has been adjusted to a pH of from9-1 1 the copper is dissolved using the ammonium citrate and preferablyan oxidant. The oxidant could be sodium nitrite, ammonium persulfate,ammonium perborate, hydrogen peroxide, and any other well-knownoxidants. Once the copper is dissolved by the formation of the chelatingcomplex, the copper can be removed by passing it through a cationexchange resin.

Through this process, the scale from this system can be removed andseparated into the cobalt complex, the iron complex, and the coppercomplex. The chelating agent also acts to pacify the metal surface andprotect the surface from further corrosion.

Through this process, the crud of a nuclear reactor has been removed andseparated into the cobalt complex which is the long-life radioactivesubstance and the iron and then the copper which are the short-liferadioactive substances. The long-life radioactive substance, cobalt,after being adsorbed on the cation exchange resin can be buried.

The cation or anion exchange resins that adsorbed the short-liferadioactive substances can be regenerated after the radioactivesubstance, iron and copper, have been dissipated. These resins areregenerated using any mineral acid or salt of a mineral acid. Preferablysulfuric acid is used because it is more efficient.

The invention can be better understood by reference to the followingtable.

TABLE I Metal pH Type of Form of Resin Resin Cobalt 1.9-3.5 cation NH,Iron and some Copper 5-7 anion OH" Copper 9-] 1 cation H CH ELATION Thechemistry of chelation is known to those skilled in the art. Briefly,chelation is a method of controlling the amount of metal ions insolution. If a compound contains two or more complexing sites which arecapable of coordinating around the metal ion, this compound is called achelating agent.

The chelating agent forms a water-soluble complex with the metal ion.Citric, gluconic and tartaric acids, ethylenediaminetetracetic acid(EDTA), nitrilotriacetic acid (NTA), and hydroxyethylenediaminetriaceticacid (HEDTA) are the most commonly used chelating agents. It has beenfound that in the practice of this invention citric acid is thepreferred chelating agent.

Oxalic acid was tried in the place of the citric acid as a chelatingagent. Because of limited solubility, the oxalic acid was replaced bythe citric acid. Tartaric acid could be substituted for the citric acid.

The ammoniated citric acid is formed by reacting one mole of citric acidwith one mole of ammonia. Ammoniated citric acid forms more solublecomplexes with metals than unneutralized citric acid.

Just as chelating agents are capable of preventing the formation of manyinsoluble metal salts, the same principles can be put to use fordissolving already-formed scale. This may be understood more clearly ifone considers that all insoluble materials have some degree ofsolubility, no matter how small. Metal salts, commonly thought of asinsoluble, are actually in equilibrium with the soluble constituentions.

If the metallic salt is removed in some fashion, then the equilibriumcompensates by the release of additional soluble ions until theequilibrium is reestablished. A chelating agent functions by removingthe ions through the formation of a chelate structure, and thus providesthe mechanism by which the insoluble material dissolves.

Citric acid is an organic acid which forms metal complexes with manydiand trivalent ions. Citric acid is an especially effective chelatingagent for copper, iron, cobalt and nickel ions. It is a white,crystalline, non-toxic compound, and is very soluble in water. When thecomplex is formed between the metal ions and the chelating agent it canbe removed by isoporous and macroporous ion exchange resins.

By the practice of the invention cobalt, iron and copper oxides can beremoved from a cooling system of a nuclear reactor. Other metallicoxides such as manganese, chromium, nickel, possibly even vanadium, andtitanium, which may be present could also be removed. All of these metalions are usually present as the oxides or as intermetallics.

This invention is unique in the fact that one complexing agent is usedto complex all of the metal ions and by a simple procedure of using ionexchange resins the metal complexes can be selectively adsorbed from thesolution. The solution can then be safely discharged.

Long-life materials are separated and removed from shortlife materials,i.e., one of the isotopes of cobalt has a half-life of 5.2 years, whilethe half-life of one of the isotopes ofiron is 45 days.

THE NUCLEAR REACTOR COOLlNG SYSTEM In the typical 2,000-gallon reactorcooling system there is about 20 pounds of scale, commonly called crud.The iron, copper and cobalt are the main constituents of the crud. Thiscrud consists of approximately 80 percent iron, 15 percent copper, andpercent cobalt. The longer the crud is allowed to accumulate, the harderit will be to dissolve.

If the amount of crud in the nuclear reactor cooling system is known theamount of citric acid needed can be calculated.

in order to dissolve the copper an oxidant should be added. The oxidantcan be selected from the group consisting ofsodium nitrate, ammoniumpersulfate, ammonium perborate, and hydrogen peroxide. The dissolutionof copper proceeds according tothe following equations:

Fe oxidant Fe compound l-"e compound Cu Ferrous compound Cu +Cu The ironacts as a catalyst and comes from the bare metal of the equipment. Avery small amount of iron is actually needed.

After the cobalt and iron are removed from solution, the solution isreturned to the nuclear reactor cooling system and the oxidant is addedto dissolve the copper.

This invention is particularly adapted to the cooling system of anuclear reactor and metallic oxide scale.

OPERATING VARIABLES The temperature can range from 70225 F. The pH todissolve the cobalt and iron and form the chelating complex can rangefrom 1.9 to 3.5. The cobalt is adsorbed on the cation exchange resinunder these conditions. The flow rate can be one gallon per minute percubic foot. Preferably, the flow rate should be 0.5 gallons per minuteper cubic foot. Once the cobalt complex from the solution has beenremoved using the cation exchange resin, the pH is raised to from 5-7with chloride-free ammonia and the iron complex is adsorbed from thesolution using an anion exchange resin. The pH is then raised from 9-1 1with ammonia, preferably an oxidant is used,

and the remaining metal complexes are removed from the solution,including copper. This removal occurs by using a cation exchange resin.

The pressure used in this method should not exceed 50 psi. At a greaterpressure, the citric acid will decompose rapidly.

THE ION EXCHANGE RESINS The ion exchange resins that were used werepolystyrene, macroporous resins. But any type of exchange resins couldbe used. Porous type resins are preferred.

The specific anion exchange resin used is No. 21 l which is a new highcapacity, high porosity, strong base anion exchange resin. This resinwas obtained by copolymerizing polystyrene and chloromethylene. This wasthen aminated using known techniques and amines, such as trimethylamineand ethanol dimethylamine. Originally developed for severe operatingconditions, No. 211 resin has been successfully adapted todecolorization and deionization of sugar syrups.

Resin 210, a high capacity cation exchange resin of medium porosity, iscommercially available as perfect, strain-free beads with the highperformance characteristics of premium quality resins. This resin iscomposed of polystyrene crosslinked with divinyl benzene. This is thensulfonated using sulfuric acid or chlorosulfonic acid. Produced fromstyrenedivinylbenzene, it is stronger, tougher, and more stable thanother conventional resins. This resin is available as Resin 210- A whichis the H form or as Resin 210-B which is the Na form.

Some of the physical and chemical properties of the specific resins usedare given in the following table.

TABLE I.PIIYSICAL AND CHEMICAL PROPERTIES Resin 211 Resin 210-A Resin210-B Physical iorm... Hard, creamllard goldenllard, goldencoloredcolored beads, colored beads. spheres, shipped moist shipped moistshipped moist in hydrogen in sodium in chloride form. form. form.Sphericity minimum (005% typical. 00-05% typical. Shipping weight" 441h./cu. it 50 lh./eu. it. 53 lb./cu. it. Moisture content. 50-60%50667,; 4-1-4892. Standard screen 2H0 mesh 16-40 mesh 16-40 mesh size.(wet). (wet). (wet). On 16 mesh 3% maximum... 15% maximunn. 15% maximum.Through 40 mesh. 3% maximum 6% maximum... 6% maximum. Through 50 mesh1.0% maximum. 1.0% maximum. Swelling Approximately 30% from the chlorideto the hydroxide forms. Salt splitting Cl" lorm.. 11+ f0rm.. Na+ form.

capacity. Meg/g. dry resin 3.7-4.3.. Meg/ml. wet 1.151.35..

resin. KgrJcu. it. as 25-29 39.0 43.5.

CaCOz. Color throw 40 maximum. 40 maximum.

APHA number.

It is not necessary to use ion exchange resins having all of theproperties listed in the above table. It would be obvious to one skilledin the art to substitute comparable ion exchange resins for those listedabove.

In the practice of this invention, it was found that some forms ofresins are preferred over other forms. For instance, to adsorb thecopper complex, it was found that resin 210-A was preferred. For theiron complex, resin 211 was used, but is commercially available only inthe chloride form. The hydroxide form is preferred. Therefore, toconvert the resin from the chloride form to the hydroxide form, from 25to 50 pounds of from 5 to 10 percent sodium hydroxide per cubic foot ofresin is used. The contact time should be at least 1 hour. The effluenthas to be tested for chloride, since chloride has a high corrosiveeffect on stainless steel. Therefore, if any chloride remains in theeffluent, there would be a large amount of corrosion when the solutionis returned to the nuclear reactor cooling system for the dissolution ofthe copper.

In order to adsorb the cobalt complex, resin 210 was used. But again aconversion is necessary. In order to convert resin 2 lO-A, which is theH form to the preferred NFL, form, from 10 to 15 pounds of ammoniumhydroxide per cubic foot of resin was used. This is a relatively simpleconversion since basically it is a neutralization in which the hydrogenand the hydroxide form water, leaving the ammonium ion on the resin. Inorder to convert resin 2l0-B which is the Na form to the NI-I, form,from 50 to 75 pounds of ammonium hydroxide per cubic foot of resin wasused. This conversion proceeds less rapidly.

Other forms of the resins could be used and comparable substitutionsshould be obvious to those skilled in the art.

In order to more thoroughly understand the invention, the followingexample is given.

EXAMPLE Over 1,300 gallons of 4 percent by weight of ammoniated citricacid was added to a nuclear reactor cooling system. This solution wasprepared by dissolving 440 pounds of citric acid monohydrate inl,300gallons of water. To form the ammoniated citric acid specie at pH 3.0,17.8 pounds of 100 percent ammonia was added to one liter of the citricacid solution. The solution was heated to 200 F. and allowed to dissolvethe iron and cobalt and some of the copper for 8 hours. The solution wasthen passed through a column of resin 210 in the ammonium form to removethe cobalt complex. The flow rate was 0.5 gal/min. About 90 g of cobaltwere removed using 5.35 ft of resin. The effluent showed 543 g of ironfrom the first column. The solution was then passed through anothercolumn ofthe same resin to remove any remaining cobalt that may havepassed through the first column.

The pH of the solution is then raised to 6.0 using 53.4 pounds of 100percent ammonia. The solution was then passed through 17.3 ft of resin211 in the OH form to remove the iron complex. This was followed by apolishing column to remove any remaining iron. Almost all of the 543 gof Fe present were removed from the solution. The solution was thenadjusted to raise the pH to 9.5 using 49.9 pounds of 100 percentammonia. The temperature of the solution was lowered to 150 F. and thenreturned to the nuclear reactor cooling system to dissolve the copper.The solution was then passed through a column containing 31.6 ft ofresin 2I0-A which is in the H form.

The effluent could then be stored for reuse. The scale had thus beendissolved, removed, and separated into the various components. Theleakage values through all columns were: 0.08 ppm for cobalt; 0.3 ppmfor iron; and 0.1 ppm for copper. Increasing the temperature was foundto decrease the leakages. For instance, at 175 F., the leakages werebelow the detectable levels of 0.01 ppm for iron and copper and 0.1 ppmfor cobalt.

Larger flow rates could be used. For instance, 1.0

gal/min/ft could probably be satisfactory.

The cation and anion exchange resins containing adsorbed iron and coppercan be regenerated. After the radioactivity is dissipated, the cationexchange resin can be regenerated using sulfuric acid. The copper hasbeen adsorbed on the cation exchange resin and exists as a copper aminecomplex Wl'llCll is easily broken up. In using sodium chloride toregenerate the cation exchange resin, a 10 percent mixture is used and aslow flow rate.

To regenerate the anion exchange resin, about 15 pounds of sodiumchloride per cubic foot of resin is used. After setting overnight, 5-10pounds of additional sodium chloride are used.

Having thus described the invention what is claimed and desired to beprotected by Letters Patent is:

l. A method of removing metallic oxide scale from the cooling system ofa nuclear reactor which comprises the steps of:

A. dissolving the scale at a pH of from 1.9 to 3.5 at a temperature offrom 225 F. in a 0.5-6 percent solution of ammoniated citric acid toform a solution of iron, cobalt, and copper complexes;

B. removing the cobalt complex from the solution by passing the solutionthrough a cation exchange resin;

C. raising the pH ofthe solution to from 5-7 with ammonia;

D. removing the iron complex from the solution by passing the solutionthrough an anion exchange resin;

E. raising the pH of the solution to from 9-ll with ammonia;

F. returning the solution to the system to completely dissolve anyremaining scale; and

G. removing the copper complex from the solution by passing the solutionthrough a cation exchange resin.

2. The method of claim 1 in which the scale is the crud from a nuclearreactor cooling system.

3. The method of claim 2 in which an oxidant selected from the groupconsisting of sodium nitrate, ammonium per-sulfate, ammonium perborate,and hydrogen peroxide is added after the pH is adjusted to from 9-1 1with ammonia to enhance the dissolution of the copper.

4. The method of claim 3 including the additional step of:

regenerating the ion exchange resins using a compound selected from thegroup consisting of mineral acids, and salts of mineral acids.

5. The method of claim 4 including the additional step of:

treating the anion exchange resin with sodium hydroxide afterregeneration.

2. The method of claim 1 in which the scale is the crud from a nuclearreactor cooling system.
 3. The method of claim 2 in which an oxidantselected from the group consisting of sodium nitrate, ammoniumpersulfate, ammonium perborate, and hydrogen peroxide is added after thepH is adjusted to from 9-11 with ammonia to enhance the dissolution ofthe copper.
 4. The method of claim 3 including the additional step of:regenerating the ion exchange resins using a compound selected from thegroup consisting of mineral acids, and salts of mineral acids.
 5. Themethod of claim 4 including the additional step of: treating the anionexchange resin with sodium hydroxide after regeneration.